The problem with influencing physical functional parameters of integrated circuits by external disturbing quantities, such as, for example, by mechanical stress in the semiconductor material of the integrated circuit used, basically occurs in all semiconductor circuits, wherein this problem is found to be particularly annoying in integrated sensors since they use a physical functional parameter of the respective sensor element in order to thereby convert a physical useful quantity to be detected into an electrical output quantity. In practice, this physical functional parameter of the integrated circuit, in particular in silicon technology, is often influenced relatively strongly by mechanical stress in the semiconductor material.
In contrast to integrated sensors, this problem occurs less often in other integrated circuits since in this case an electrical input signal is almost always converted into an electrical output signal so that the absolute quantity of the physical parameter of the integrated circuit is not essential for its function, but only relative quantity ratios, cf. pair-tolerance, matching, of circuit parts associated to one another are relevant.
Integrated sensor assemblies, such as, for example, Hall probes, including their control and evaluating electronics (ASICs, ASIC=Application Specific IC), are increasingly used in large numbers in many applications, such as, for example, in the automobile industry, as current counters or in ventilator motors. Over the last few years it has also become possible in integrated circuit technology to separate, with the principle of the spinning current Hall probe, a disturbing offset signal of a Hall probe from the useful signal of the Hall probe by means of a mostly time-discrete signal processing.
With these integrated sensor assemblies, the possibility arises to manufacture very precise magnetic field sensors with extended additional functions, such as, for example, the possibility of programmability and so-called “smart sensors”, in large numbers in a reliable and cost-effective way in reliable CMOS or BiCMOS technology with silicon as the semiconductor basic material. In particular due to this combination of advantages, discrete Hall probes, which were preferably used at some point, including their control circuits including direct semiconductor materials, such as, for example, GaAs and InSb, are being employed to an ever-decreasing extent. Direct semiconductor materials are such semiconductors in which the energy maximum of the valence band and the energy minimum of the conduction band are at identical crystal impulses.
Thus, certain disadvantages of indirect semiconductor materials, such as, for example, silicon or germanium, are becoming more and more evident, wherein indirect semiconductor materials are such semiconductors in which the energy maximum of the valence band and the energy minimum of the conducting band are at different crystal impulses. In indirect semiconductor materials, so-called piezo effects can be observed relatively frequently. In this context, piezo effects are changes of electrical parameters of the semiconductor material under the influence of a mechanical stress in the semiconductor material. With piezo effects, it is in particular differentiated between the piezo-resistive effect and the piezo-Hall effect in a semiconductor material.
The piezo-resistive effect indicates how the specific ohmic resistance ρ of the respective semiconductor material behaves under the influence of a mechanical stress tensor, wherein the following applies:ρ=ρ0(1+Σπi,jσi,j)
The piezo-Hall effect, in contrast, describes the dependence of the Hall constant on the mechanical stress condition in the semiconductor material, wherein the following applies:Rh=Rh0(1+ΣPi,jσi,j)
Thus, σi,j is the mechanical stress tensor, πi,j are the piezo-resistive coefficients, Pi,j are the piezo-Hall coefficients, wherein the summation extends over i=1 . . . 3 and j=1 . . . 3.
Both effects are disturbing when operating a sensor assembly, such as, for example, an integrated Hall probe including control and evaluating electronics. The current-related sensitivity Si of the Hall probe, for example, changes by the piezo-Hall effect depending on how this functional parameter of the sensor assembly is influenced due to changes of the mechanical features of the sensor casing, wherein the following relation applies for the current-related sensitivity Si of the Hall probe:       S    t    =                    U        h                              I          H                ⁢        B              =                            R          h                t            ⁢      G      
Thus, Uh is the Hall voltage on the output side of the Hall probe, IH is the current through the Hall probe, B is the magnetic flux density, t is the effective thickness of the active layer of the Hall probe and G is a geometry factor describing the influence of the contact electrodes on the Hall voltage.
In addition, the Hall current through the Hall probe changes as a consequence of the piezo-resistive effect when applying mechanical stress in the semiconductor material, since the Hall current is, for example, only defined via a resistor across which there may be a voltage in a control loop. A change of the Hall current due to a resistance change thus results in a change of the sensitivity S of the Hall probe, since it is identical to the product of current-related sensitivity Si times the Hall current IH:S=SiIH=UhIB
In integrated sensor circuits housed in a casing, it is to be kept in mind that the casting compound of the casing generally has a different thermal expansion coefficient than the semiconductor material, such as, for example, the silicon chip, which is why the two components can tense up relative to each other at different temperatures similar to a bimetal strip. Tensile and compressive stress components occurring in the semiconductor material can easily reach an order of magnitude of 100 MPa(1 Pa=1 N/m2) and can even result in mechanical damage of the chip, i.e. cracks on the surface of the chip or even breaking of the chip.
In the present invention, the focus of attention, however, is not directed to potential damage by mechanical stress in the semiconductor material but basically to the influence of physical parameters of the semiconductor material by these mechanical stress which can, for example, influence the electrical and magnetic characteristics of an integrated sensor assembly, such as, for example, a Hall sensor assembly.
The mechanical stress in the semiconductor material can, comparable to an elastic shape change work, be considered as a type of excitation energy which has to be added to the energy balance in the semiconductor material. In particular, mechanical stress in the semiconductor material results in a change of the band structure of the semiconductor. In indirect semiconductor materials, the result is a splitting of energy minima which are actually identical in a stress-free casing. As a further consequence, this causes a reoccupation of these energy minima with free charge carriers, wherein a predominant portion of the charge carriers will accept the state which, as far as energy is concerned, is more favorable. Since the bending of the band edges, i.e. the edges of the energy bands of the free charge carriers in the semiconductor crystal, i.e. the conduction band edge for the free electrons and the valence band edge for the free holes, is also different in the different energy minima, a different effective mass can be associated to the charge carriers in these energy minima, which is why their behavior differs as far as the charge carrier transport is concerned. In this way, mechanical stress in the semiconductor material has the effect that the features of the charge carriers change as far as the charge carrier transport is concerned, such as, for example, mobility, collision time, scattering factor and Hall constant.
So far, it has been impossible to keep this mechanical stress in a defined predetermined region during the entire lifetime and the entire temperature range of a sensor assembly. In addition, what aggravates the situation is that magnetic field sensors are cast in particularly thin casing types so that they can be inserted into small air gaps in the respective case of application since the narrower the air gaps, the higher the magnetic field in that gaps. No gel can be applied to the semiconductor chip supporting the sensor assembly for such thin casing types due to a lack of space, as is otherwise often employed to cast the semiconductor chip with little stress. In relation to the piezo-resistive effects in silicon, reference is, for example, made to the scientific publication by Yozo Kanda “A graphical representation of the piezo-resistive coefficients in silicon” in IEEE Trans. Electron Devices, Vol. ED-29, pp. 64-70, January 1982. In connection to integrated Hall probes, there are some studies indicating that a large part of the offset signal of these Hall sensors can be explained by the piezo-resistive effect, for this see U.S. Pat. No. 5,614,754 “Hall device”, Aug. 2, 1994 by Kazuhiko Inoue and U.S. Pat. No. 4,025,941 “Hall element”, Apr. 8, 1975 by Yozo Kanda et al.
In addition, there are extensive studies regarding the directional dependence of the piezo-resistive effect in n-and p-doped silicon for both light dopings below 1017 cm−3 and for heavy dopings up to 1023 cm−3, for this see “Piezo-resistance effect in germanium and silicon”, Physical Rev., Vol. 94, pp. 42-49, 1954 and “Piezoresistive Properties of Heavily Doped n-Type Silicon” by O. N. Tufte and E. L. Stelzer, Physical Rev., Vol. 133, No. 6A, pp. A1705-1716, Mar. 16, 1964. In this regard, there is a well-established theory confirming the measuring results obtained by experiments quantitatively on the basis of the band structure of the semiconductor, see “Transport and deformation-potential theory for many-valley semiconductors with anisotropic scattering” by C. Herring and E. Vogt, Physical Rev., Vol. 101, pp. 944-961, 1956. In relation to studies and theoretical explanations of the piezo-Hall effect in n-doped silicon, reference is made to the scientific publication “Piezo-Hall coefficients of n-type silicon” by B. Halg, J. Appl. Phys., 64 (1), pp. 276-282, Jul. 1st, 1988.
The patent U.S. Pat. No. 4,929,993 relates to, for example, a Hall device having a blocking layer protection barrier. Referring to FIG. 17, it is illustrated there that the circuit, apart from a Hall element 22 and a current source 21, also comprises a control circuit assembly 24, 25, 26, 27. The thickness of the blocking layer 11 is to remain constant even when disturbing influences, such as, for example, a change of the ambient temperature, occur. In order to achieve this, the Hall element illustrated in FIG. 17 is to be switched always with the control circuit assembly 24, 25, 26, 27 keeping the depth of the blocking layer at a correct value. In this context, the field-effect transistor 32 serves as a temperature-sensitive element, the pinch-off voltage of which is inversely proportional to the square of the ambient temperature. The temperature of the Hall element 22 and of the field-effect transistor 32 are actually identical since these two components form a part of the same integrated circuit and are thus very close to each other. The control circuit assembly 24, 25, 26, 27 controls the thickness of the blocking layer in the Hall element 22 by comparing the output voltage of the actual value generator 24 to the set value supplied by the set value generator 25 by means of the difference generator 26, 27. The sum of the difference obtained between the set value and the actual value is supplied to the control input M of the Hall element 22. Since the field-effect transistor 32 is a temperature-sensitive component, the set value depends on the temperature. Thus, it is made possible for the control circuit assembly 24, 25, 26, 27 to regulate the thickness of the blocking layer to a value allowing the magnetic field sensitivity of the Hall element 22 to be set independently of the temperature.
The patent EP 1010987 A2 relates to, for example, a Hall sensor having means for orthogonally switching the Hall sensor supply current and the Hall voltage taps, wherein the geometry of the Hall plate, in the orthogonal position, is the same for the Hall voltage determination, and summing means which the Hall voltage values of the orthogonal positions can be supplied to for forming an offset-compensated Hall voltage value. Optionally, a stress measurement and a temperature measurement can be performed for a conventional magnetic field measurement. For a stress measurement, the Hall voltage values of the orthogonal positions are to be supplied to summing means and/or processible in summing means in such a way that the portions of the Hall voltage values due to the magnetic field compensate and only portions of the Hall voltage values due to the offset can be measured.
The patent U.S. Pat. No. 5,686,827 relates to a Hall effect device generating an electrical output signal when it is subjected to a magnetic field. An operating current for the Hall effect device is varied by an input circuit assembly in order to keep the output signal of the Hall effect device constant as regards variations of the temperature of the Hall effect device, while the magnetic field applied to the Hall effect device remains constant. The input circuit arrangement includes a temperature-sensitive element kept at a temperature varying as a function of the variations of the temperature of the Hall effect device. The temperature-sensitive element is embodied as a silicon diode.
The patent DE 3932479 A1 describes an arrangement for processing sensor signals. The arrangement serves for processing sensor signals provided by a measuring sensor and by one or several more sensors. The measuring sensor produces a measuring effect depending on a physical measuring quantity to be detected and on physical disturbing quantities, wherein each additional sensor generates a measuring effect basically only depending on physical disturbing quantities. The arrangement contains a signal processing circuit receiving the measuring effects as input quantities. The signal processing circuit, by analogue processing of the sensor signals, provides an output quantity, the relation of which to a reference quantities is determined by a transfer function defined in a special way in dependence on the measuring effects forming the input quantities. The coefficients of the transfer function can be set for obtaining the desired transfer behavior independence on the features of the measuring sensor, which is how the influence of the disturbing quantity on the measuring effect on the measuring sensor is compensated and errors are corrected in the relation between the measuring quantity and the measuring effect of the measuring sensor.